Apparatus and method for spinning hollow polymeric fibres

ABSTRACT

A method of manufacture of solid-walled hollow polymeric fibres comprises the steps of dissolving polymeric material in a suitable solvent liquid to form a dope solution (44), extruding the dope solution through an aperture in a spinneret (41) to form a narrow jet of liquid injecting a coagulant (46) through an aperture (53) in the centre of the liquid dope jet as it leaves the spinneret, directing the jet through an air gap into a coagulant bath containing a further coagulant; and directing the fibre through a drawing bath to reduce the diameter, each coagulant solution being a mixture of a coagulant liquid capable of causing gelation and solidification of the liquid dope jet and between 20% and 80% of the solvent liquid.

The invention relates to methods of manufacture of hollow polymericfibres by wet spinning, to a multi-hole spinneret for use in suchmanufacture, and to a method of production of hollow carbon fibre fromhollow polymeric fibre, specifically polyacrylonitrile.

BACKGROUND OF THE INVENTION

Spinning has been defined as the transformation of a liquid materialinto a solid fibre. There are three main methods for spinning fibres:melt spinning, dry spinning and wet spinning. These methods can becombined depending on the final properties required of the material(such as a polymer) being spun.

Melt spinning is preferred if the polymer can be melted withoutdegradation and is a common method for spinning thermoplastics such aspolypropylene and nylon. The molten polymer is extruded through aspinneret into a gaseous medium such as air where the fibre coolsproducing solid, non-porous fibre. The filament is usually then drawn toorientate the polymer molecules which also improves the tensileproperties of the fibre.

Dry spinning involves the extrusion of a polymer dope (polymer dissolvedin an appropriate solvent) into a heated zone where the solventevaporates. This is a slower process than the cooling of melt spunfibres and, as a result tends to produce fibres with non-uniformproperties and a less circular cross section.

Wet spinning is identical to dry spinning except in the way the solventis removed from the extruded filaments. Instead of evaporating thesolvent, the fibre is spun into a liquid bath containing asolvent/non-solvent mixture called the coagulant. The solvent is nearlyalways the same as that used in the dope and the non-solvent is usuallywater.

Dry and wet spinning can be combined to form a process known as dry jetwet spinning. Polymer dissolved in a suitable solvent is extruded into agap before entering a coagulation bath containing a coagulant that ismiscible with the solvent but not with the polymer. A phase inversionprocess takes place producing a solid fibre. The bath can contain amixture of solvent and non-solvent. This method helps prevent blockageof the spinneret and also allows some drawing of the fibre prior tocoagulation, increasing orientation of the polymer molecules. The airgap has been shown to produce fibres that are stronger and moreextensible than fibres produced from an immersed jet.

The fibre microstructure is established in the coagulation bath andrequires optimisation of conditions. The critical process is thetransition from a liquid to a solid phase within the fibrils and thereare two possible such transitions. One is phase inversion--theprecipitation of polymer to form a solid phase, the other is gelation.The former yields fibre of poor mechanical properties where as thelatter produces an elastic gel giving rise to a fine microstructure oncethe solvent is removed. For membrane-type fibres phase inversion ispreferable. For fibres with the appearance of a solid wall phaseinversion should be slowed down so that gelation precedes phaseinversion. Conditions in the coagulation bath have, therefore, to beoptimised so that gelation precedes phase inversion. It has been shownthat gelation occurs more rapidly at lower temperatures and at highersolid concentration in the dope.

The concentration of solvent in the coagulation bath can also beadjusted to obtain the desired microstructure. A low solventconcentration promotes rapid solvent extraction although this results ina thick skin on each filament which ultimately reduces the rate ofsolvent extraction and can lead to the formation of macrovoids. A highconcentration of solvent in the coagulant gives a denser microstructurebut solvent extraction is low. Temperature of the coagulation bath, jetstretch and immersion bath can similarly affect coagulation andmicrostructure. The fibre produced is essentially a swollen gel and isunoriented. The microstructure consists of a fibrilar network with thespaces in-between called macrovoids.

The invention is directed towards an improved spinning method of dry-jetwet spinning which enables the production of hollow polymeric fibreswith the hole or lumen accurately centred and permits an enhanced degreeof control over the wall properties. Consistent wall properties arelikely to be of great significance in a range of applications: forexample the best combination of tensile properties is achieved when thefibre has a homogeneous, dense gel structure with small fibrils and nomacrovoids; for application as a membrane the wall ideally has a highlyoriented inner and outer skin separating a porous body. The invention isalso directed towards a suitable spinning apparatus; in particular onewhich is suitable for the production of polyacrylonitrile fibressuitable for subsequent processing to produce hollow carbon fibres.

According to an aspect of the invention a method of manufacture ofhollow polymeric fibres comprises the steps of:

i) dissolving polymer in a suitable solvent to form a dope;

ii) extruding the dope through an aperture in a spinneret to form a jetof liquid;

iii) injecting a first coagulant into the centre of the dope jet as itleaves the spinneret;

iv) directing the jet through an air gap into a coagulant bathcontaining a second coagulant such that a fibre is formed;

v) directing the fibre through a drawing bath to reduce the diameter;wherein each coagulant comprises a mixture of a coagulant liquid capableof causing gelation and eventual solidification of the dope jet andbetween 20% and 80% of the solvent liquid.

The invention produces hollow fibres whilst allowing a high degree ofcontrol over the spinning conditions and thus over the structure of thefibre wall. In particular for fibres with the appearance of a solid wallphase inversion should be slowed down so that gelation precedes phaseinversion. The hollow fibres thereby produced offer comparable tensileproperties at reduced weight in comparison to solid fibres produced byconventional wet spinning, offering advantages in a range ofapplications such as in the production of hollow fibres for textiles. Itwill be understood that the invention is not limited to production ofsingle fibres but can produce multiple fibre arrays from multiple liquidjets either by providing a spinneret with multiple apertures or byproviding an array of spinnerets.

Carbon fibres are manufactured by pyrolysing organic precursor fibres,predominantly polyacrylonitile (PAN) fibres produced by wet spinning. Itmay be noted here that the polyacrylonitrile fibre is used in this artto include co-polymers or ter-polymers of acrylonitrile with othermonomers. For precursors of carbon fibre this is typically a copolymerwith itaconic acid which controls the cylcisation reaction duringpyrolysis. The requirement that gaseous products must be able todiff-use through the fibres from the surface to the centre, andvice-versa during the oxidation and carbonisation processes, imposes anupper diameter limit and the technique is limited to the production ofcarbon fibres for structural applications with diameters up to about 10μm.

In the last decade, the tensile strength of these fibres has beendoubled, leading to large increases in all tensile-related compositeproperties. However, under compressive loading the failure process ismicro-buckling. Compressive strength is therefore strongly influenced bythe diameter limit set by the manufacturing process and has remainedlargely unchanged over this period. As a result this property is oftenthe key design parameter in strength critical applications. Hollowcarbon fibres offer a possible solution as they offer the potential forincreased second moment of area and hence resistance to buckling withoutexceeding thickness limits. This would require production of hollowprecursor fibres of an appropriate size, and with a dense walledstructure without macrovoids.

The invention is thus particularly applicable to the production ofacrylic fibres such as polyacrylonitrile to serve as hollow carbon fibreprecursors. Polyacrylonitrile of molecular weight in the range 80,000 to200,000, typically about 120,000 is preferred, and is dissolved in anappropriate aprotic solvent, of which dimethyl formamide (DMF) andsodium thiocyanate are non-limiting examples. The dope formed preferablycontains between 15% and 30% by weight, and typically 25%, by weight ofpolyacrylonitrile in the appropriate solvent. A preferred coagulant iswater. The polymer concentration in the dope solution is preferably inthe range 15-25%. The solvent concentration in the coagulant solution ispreferably in the range 30-60%.

There is also the potential to incorporate a third phase into the hollowfibre core after formation which could find application in the smartmaterials field. For example, uncured resin could provide in-situ repaircapability after fibre fracture or suspensions of fine powders could actas radar absorbers for stealthy capability.

Hollow carbon fibres suitable for applications where conventional carbonfibres are used at present will have diameters in the preferred range20-40 μm, corresponding to polyacrylonitrile precursor fibre diameter ofaround 30-65 μm, with a wall thickness of 5-10 μm. Diameters of hollowcarbon fibre in the region of 25 μm from polyacrylonitrile fibres ofdiameters in the region of 40 μm are particularly preferred. Fibrediameters are controllable through the aforementioned spinningvariables. The process preferably requires stretching in a heated zoneto reduce the spun fibre to the required diameter. The drawing bathconveniently contains heated liquid to facilitate this. Embrittlementthat may ensue due to orientation effects and can adversely effectproduction of carbon fibre can be eliminated by relaxation at raisedtemperatures.

The conversion of the hollow PAN precursor to a hollow carbon fibre isachieved via the pyrolysing process which is used for solid carbonfibres and which will be familiar to those skilled in the art.

Another aspect of the invention provides a spinneret for manufacture ofhollow polymeric fibres, and in particular hollow polyacrylonitrileprecursors for carbon fibres, comprising a hollow body, a first inletfor a dope, a second inlet for a coagulant, a base plate having at leastone extrusion aperture for extrusion of the dope, and coagulantinjection means to inject a coagulant into extruded dope solutionalignable to the centre of the or each extrusion aperture and incommunication with the second inlet, such that in use a stream of dopeis extruded through the or each aperture having a stream of coagulant atits centre. Each injection means conveniently takes the form of a hollowneedle in communication with the second inlet and provided with anaperture at one end which can be aligned with the centre of anassociated extrusion aperture.

To control the flow parameters, the injection means is preferablyprovided with vertical microadjustment means to control the distancebetween it and the extrusion aperture. Lateral microadjustment means toensure accurate centring of the injection means over the extrusionaperture are also preferred.

At its simplest, this aspect of the invention comprises a singleextrusion aperture and a single injection means. In the alternative, thebase plate is provided with a number of extrusion apertures and thespinneret further comprises a number of injection means alignable to thecentre of the extrusion apertures to enable multiple fibre spinning froma single spinneret. In a preferred arrangement, the spinneret has ahollow body cavity divided by an upper plate incorporating the injectionmeans into an upper portion communicating with the first inlet and alower portion communicating with the second inlet. The upper plate ispreferably provided with a number of hollow needle-like depressionsprotruding towards the base plate and alignable to the centre of theextrusion apertures.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only withreference to the polyacrylonitrile/dimethyl formamide (DMF) /watersystem and to FIGS. 1-10 in which:

FIG. 1 is a schematic of the filtering and pumping stage

FIG. 2 is an axial cross-section of a multiple spinneret for use inspinning multiple continuous hollow fibres in accordance with theinvention

FIG. 3 is a plan view from below of the spinneret of figure2

FIG. 4 is a perspective of the spinneret of FIG. 2

FIG. 5 is a cross-section of extrusion aperture profiles for injectionof coagulant

FIG. 6 is a schematic of the hollow fibre coagulation and stretchingapparatus

FIG. 7 is a scanned image of a scanning electron photomicrograph showinga hollow carbon fibre produced from a hollow polyacrylonitrile precursor

DETAILED DESCRIPTION OF THE INVENTION

Polyacrylonitrile of molecular weight in the range 80,000 to 200,000,typically about 120,000 is dissolved in dimethyl formamide (DMF). Thedope formed contains approximately 25%, by weight, of polyacrylonitrilein the solvent. This percentage is attained by rotary evaporation from alower concentration. In the particular systempolyacrylonitrile/DMF/water, a minimum grade of purity of the DMF isrequired--this is specified as technical grade of minimum assay (GLC) of99%. The resultant dope will be moderately viscoelastic with a zeroshear viscosity in the range 50-300 Pa.s at 20° C., and typically about120 Pa.s. It is also possible for the viscosity of the spinning dope tobe reduced by heating.

The dope is then filtered to ensure that flow through the spinneretremains unrestricted, FIG. 1. This is typically achieved by forcing itunder nitrogen pressure (through nitrogen feed 6) of typically 6 barthrough an on-line filter, 2, in which a 40 μm stainless steel meshstrainer is typically used. The dope is then pumped via a pump 3 througha second on-line filter 4, in which a 5 to 20 μm sintered stainlesssteel filter is typically used, and is then passed to the spinneret 41.

A spinneret arrangement is illustrated in FIGS. 2 to 4. The dope andcoagulating liquid are injected into the spinneret, 41, at separatelycontrollable rates via one or more inlet pipes 42 and 43 respectively.The dope passes into a lower body cavity 44 of the spinneret and thecoagulant liquid is channelled through an upper body cavity 46. Thecavities 44 and 46 are separated by an upper plate 51 which is providedwith a plurality of downwardly extending extrusions 52 each ending in anaperture 53 which communicates with the upper body cavity 46 and throughwhich a jet of coagulant is extruded into the dope jet. The protrusions52 thus provide injection means for the coagulant. Alignment of baseplate 48 to the protrusions 52 is then performed so that each aperture49 serves as an outer annulus 50 which communicates with the lower bodycavity 44 and through which the dope jet is extruded, with coagulantextruded through the inner aperture 53. This can be achieved opticallythrough the use of a laser beam and the base plate thence mechanicallyfixed, or, for example, through the use of the well-known mechanism ofcentring screws 54.

Typical dimensions to enable production of fibres for structuralpurposes are from 220 μm to 600 μm (inner diameter) of aperture 53, 100to 300 μm outer diameter of the protrusions 52, and inner diameter50-200 μm. It will be appreciated however that the invention is notlimited to this area and is applicable to production of hollow fibresfor utilisation in other areas, in which case dimensions may be changed,for example, an inner diameter of aperture 53 of 1 mm would be typicalfor membranes. Examples of injection profiles are illustrated in FIG. 5.

As FIG. 6 illustrates, the resulting stream of dope and coagulant 20 ispassed from the spinneret 41 through an air gap into a coagulating bath22. The air gap (from spinneret to surface of the bath) is preferablybetween 8 and 30 cm, but ideally from 10-15 cm. Beyond 30 cm the streamof dope is unstable and unsuitable for processing.

Different structures can be obtained by control of the temperature ofthe coagulating bath and through variation of the proportion ofcoagulant to solvent. To produce fibres with the appearance of solidwalls, coagulation must be slowed down whilst keeping diffusion rateshigh. This is ensured by the addition of solvent to conventionalcoagulants to such a level as to form a coagulant solution under theaction of which the formation of the outer skin is slowed down comparedwith conventional coagulant liquids alone. Practical levels of solventaddition in the coagulant solution are in the range 20-80%, preferablyin the region 30-60%. For example, for the system polyacrylonitrile/DMF/water the coagulation bath contains a solution 24 comprising 1:1 byweight of water:DMF cooled to between 4° C. and 9° C., but typically 8°C. ±1° C. To prevent the fibre flattening as it passes around therollers and to maintain a circular cross-section, it has to be allowedto sufficiently solidify to impart a degree of rigidity. This isachieved by passing it round a lead guide 25, of diameter not less than4 cm diameter, at least 0.5 m and a maximum of 1.5 m below the surfaceof the coagulation bath. The guide has a mechanism for raising andlowering it into the coagulation bath.

The fibre 21 is then directed via further guide rollers 26 which may, ormay not, be driven onto a motor driven guide roller 27. Variation of thedrive rate of the roller 27 can be used to vary the speed at which thefibre 21 is drawn through the coagulating bath to control the jetstretch and orientate the fibre.

A bank of filter units is fitted along the coagulation bath to providelaminar air flow for withdrawal of potentially hazardous fumes, forexample when using DMF. To reduce impurities within the fibres cleanroom conditions should be utilised. Such impurities are known to have adeleterious effect on resultant carbon fibre properties and the use ofan anteroom for entrance to the spinning environment and air filtrationhas been demonstrated to reduce such effects.

The fibre 21 is then passed into a heated zone between 95-100° C. toreduce diameter and to impart a degree of orientation. This maytypically be a bath, 30, of water, 32, heated to near boiling point. Thefibre passes via further guide rollers 28 onto a further driven roller29. As before, variation of the drive rate of the driven roller 29 canbe used to effect stretching of the fibre thereby, reducing thediameter. The rollers 28 are provided with a mechanism to be raised outof and lowered into the water 32. The fibre is then passed to acollecting drum in a washing bath 34. Subsequent washing may be dynamicor static for a minimum of 48 hours, though this is less critical if thefibre is to be pyrolysed.

The conditions under which the fibres are spun have influence on theirfinal properties. Fibre diameter is ultimately controlled by the size ofthe aperture 53 through which they are extruded but post extrusionstretching, or drawing, of the fibres can also affect the finaldimensions. The amount of post extrusion stretching also effects thetensile properties of the fibre.

As a measure of the amount of stretching that a fibre has receivedduring its extrusion, the dimensionless term "Jet Stretch" (JS) isnormally used and is defined as:

    JS=A.sub.SP V.sub.f /DER

where V_(f) is the fibre velocity (mm s⁻¹) on the first take-up roller,A_(SP) is the annulus area of the spinneret (mm²) and DER is the DopeExtrusion Rate (mm³ s⁻¹) from the spinneret.

The amount of stretching that a fibre receives in the heated stage isthe ratio of the fibre velocity on the roller at the start of the heatedstage (V_(fstart)) to the fibre velocity on the roller at the end of theheated stage (V_(fend)) and is given the term "Draw Ratio" (DR):

    DR=V.sub.fend /V.sub.fstart

With known values of the velocities of the rollers, the diameters of theorifice plate and the needle diameter, the dope extrusion rate and theperfusor rate, it is possible to estimate the diameter of the fibre andthe diameter of the lumen on the final roller. A typical example isshown in Table 1. An example of different jet stretches and influence ontensile properties is given in Table 2.

                  TABLE 1                                                         ______________________________________                                        Determination of approximate fibre dimensions                                 Parameter     Symbol/formula  Typical value                                   ______________________________________                                        Perfusor rate PR              50     μl min.sup.-1                         Orifice diameter                                                                            ORI             600    μm                                    Needle outer diameter                                                                       NOD             305    μm                                    Annulus area  Ann =               2.1 × 10.sup.-5 m.sup.2                                     (ORI.sup.2 -                                                                  NOD.sup.2)/4                                            Fibre velocity (first roller)                                                               VF              130    mm s.sup.-1                              Fibre velocity (last roller)                                                                VL              380    mm s.sup.-1                              Dope concentration                                                                          DC              25%                                             Dope extrusion rate                                                                         DER             4.5    mm.sup.3 s.sup.-1                        Jet stretch   JS =    VF.Ann/DER  1.71                                        Draw ratio    DR =    VL/VF       2.92                                        Jet-Draw function                                                                           JR =    JS.DR       4.99                                        Fibre diameter                                                                              r.sub.1 =                                                                             (4.(PR +    81.0 μm                                                        DC.DER)/                                                                      .DR.VF)                                                 Lumen diameter                                                                              r.sub.2 =                                                                             (4.PR/.DR.VF)                                                                             52.9 μm                                  ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Examples of effect of chaniging the draw ratio                                     fibre    fibre           strain                                                                             Energy                                          outer    inner     Modu- at   to    Tenacity                             draw diameter diameter  lus   break                                                                              break at break                             ratio                                                                              (μm)  (μm)   (N/Tex)                                                                             (%)  (mJ)  (N/Tex)                              ______________________________________                                        3.23 60       47        5.08  18.44                                                                              4.27  0.172                                3.91 66       51        6.46  14.86                                                                              3.29  0.236                                4.91 63       43        7.53  13.24                                                                              2.44  0.267                                5.96 57       35        9.02  12.46                                                                              1.99  0.308                                ______________________________________                                    

The conversion of hollow polyacrylonitrile precursor to hollow carbonfibre is achieved via the usual three stage process of oxidation,carbonisation and graphitization which is used for solid carbon fibresand which will be familiar to those skilled in the art. The fibres areheated in an oxygen containing atmosphere between 200° and 300° C.whilst under tension so as to prevent shrinkage and even causeextension. The chemistry of the process is very complex and will befamiliar to some of those skilled in the art. Two important processesare the reaction of nitrile groups to form ring structures and promotionof cross-linking by oxygen. The former is particularly exothermic andmust be performed at a controlled rate. This may be achieved through avariety of methods, for example passing through a series of four ovenswith progressively increasing temperatures in the temperature rangespecified. Oxidation stabilises the fibres for the subsequentcarbonisation step. Carbonisation is carried out in an inert atmosphere,typically nitrogen, at approximately 1000° C. for commercial processesto remove non-carbon elements as volatiles; a non-exclusive listincludes H₂ O, HCN, NH₃, CO, CO₂ and N₂. The rate of heating in theearly stages is generally low so that the release of volatiles does notdamage the fibre. This may typically be achieved by passing the fibrethrough a furnace with a gradual temperature gradient from above 350° C.to 700-1000° C. The resultant carbon fibre has lost most of itsnon-carbon impurities. Further heat treatment at temperatures in therange 1300-3000° C. can improve mechanical properties; Young's modulusis clearly related to the final heat treatment temperature ofgraphitization. Further changes in processing, for example theapplication of tension during carbonisation and graphitization caneffect mechanical properties. An example of a resultant hollow carbonfibre is shown in FIG. 7.

What is claimed is:
 1. A hollow polyacrylonitrile fiber precursor for hollow carbon fiber, the precursor polyacrylonitrile fiber comprising a central lumen and a wall having a highly oriented interior surface facing the central lumen, a highly oriented exterior surface facing away from the central lumen and a homogenous, dense gel structure between the interior surface and the exterior surface, wherein the entire hollow polyacrylonitrile fiber is free of macrovoids.
 2. A fiber in accordance with claim 1 having a diameter of 30 to 65 μm.
 3. A fiber in accordance with claim 2 having a diameter of about 40 μm.
 4. A fiber in accordance with claim 1 having a wall thickness of 5 to 10 μm.
 5. A fiber in accordance with claim 1 comprising a copolymer of acrylonitrile with italonic acid.
 6. A hollow polyacrylonitrile fiber precursor for hollow carbon fiber, the precursor polyacrylonitrite fiber comprising a central lumen and a wall having a highly oriented interior surface facing the central lumen, a highly oriented exterior surface facing away from the central lumen and a homogenous, dense gel structure between the interior surface and the exterior surface, wherein the entire hollow polyacrylonitrile fiber is free of macrovoids, said hollow fiber produced by the method of:(a) dissolving acrylic polymer in a solvent to form a dope; (b) extruding the dope through an aperture in a spinneret to form a jet of liquid; (c) injecting a first coagulant into the center of the liquid jet as it leaves the spinneret; (d) directing the jet through an air gap into a coagulant bath containing a second coagulant to form a fiber; and (e) directing the thus-formed fiber through a drawing bath to reduce the fiber's diameter; wherein each coagulant comprises a mixture of a coagulant liquid capable of causing gelation and eventual solidification of the dope jet and between 20% and 80% of the solvent liquid. 